As temperatures increased above 50°F (10°C) in several large U.S. cities, risk of kidney stones also increased, researchers said.

Action Points

There is a graded increase in the risk of patients seeking medical care for kidney stones as average daily temperatures increase, and the time between hot days and kidney stone presentation is short.

As temperatures increased above 50°F (10°C) in several large U.S. cities, risk of kidney stones also increased significantly researchers said.

A study of 60,433 privately insured patients across five cities -- Atlanta, Chicago, Dallas, Los Angeles, and Philadelphia -- found that the maximum risk for kidney stone presentation occurred within 3 days of a high daily temperature and was likely mediated by an effect on patients' hydration.

The five cities represent climates in which 30% of the world's population lives, according to study author Gregory Tasian, MD, MSc, at the University of Pennsylvania, and colleagues.

Using a time series design and distributed lag nonlinear model, researchers collected private health insurance claims data from 2005-2011 as well as weather data for the selected cities. Tasian and colleagues examined presentation for kidney stones within a 20-day window of temperature exposure.

Cases tended to occur within a few days after episodes of extreme temperature, with a first peak at about 2 to 3 days and a second at 4 to 6 days.

"We were expecting to find a short lag time between heat and presentation, so it wasn't really surprising that the lag time was detected within a week," Tasian told MedPage Today.

Tasian and colleagues hypothesized that dehydration is the causal mechanism between the effect of heat and stone presentation. When patients who are already at risk get dehydrated, calcium and uric acid become more supersaturated, and calcium stones begin to form, they said.

"It's all linked to fluid. Saying heat leads to fluid loss would be the direct link," said Allan Jhagroo, MD, a professor of nephrology at the University of Wisconsin who was not associated with the study.

The researchers also hypothesized that the hotter weather may have led to stone formation in patients exposed to hotter weather who would have normally developed stones at a future time.

Colder weather was associated with a relative risk in Atlanta, Chicago, and Philadelphia, perhaps because patients stay indoors where it can be hotter. It's also conceivable that hydration may suffer during extremes of cold (when indoor humidity, which was not measured in the study, is usually low) as well as hot weather.

Outdoor humidity was measured, but was not found to be a predictor for kidney stones.

The researchers also suggested that the number of hot days in a year is probably a better indicator of kidney stone risk than mean annual temperature. Atlanta, for example, had almost twice the rate of kidney stones compared with Los Angeles but had a similar mean temperature. It had, though, on average 53 days a year in which the daily mean temperature was higher than 80°F. Los Angeles had only 10.

Dallas had 324 days hotter than 86°F during the period, 20 times more than such days in Atlanta, the next closest city. But it had the same risk increase. Tasian and colleagues suggested that the population of Dallas may have adapted to the local climate, spending more time indoors and drinking more fluids. They also noted that their data were sparse for extreme weather, and their statistical methods may have flattened the associations somewhat.

A previous study reported by MedPage Today found that, as temperatures across the U.S. increase because of climate change, the prevalence of kidney stones may be expected to grow. Tasian said that more research needs to be done to see how the risk of kidney stones may change with temperature increases.

The authors acknowledged several limitations to the study. They had no data on individuals' actual exposure to outdoor temperatures, which would vary. All the patients had commercial insurance and may spend more time indoors, with air conditioning, than those with public or no insurance. It is also possible that temperature differentially affects subgroups such as older versus younger patients.

Additionally, the research was concerned only with presentation at the hospital and not stone formation.

Jhagroo suggested that this left open the question of whether "warmer weather leads to passage, or warmer weather leads to both formation and passage."

The study was funded by grants from the NIH.

The authors disclosed no relevant relationships with industry.

Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania

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